This invention relates to alarm devices and, more particularly, to alarm devices intended to be installed on an automotive vehicle in order to signal a substantial drop in the inflation pressure of the tires on the wheels of the vehicle. The invention relates with still greater particularity to a device comprising an electric circuit having means detecting a change in the condition of the circuit and a flexible rod one end of which is fastened to a fixed, unsuspended part of the vehicle, the free end of the rod being directed towards the ground.
There are already known devices of the general type described above. However, in these known devices, the inclination of the rod varies with respect to the ground when its free end touches the ground. If the variation in inclination exceeds a given value, this causes a change in the condition of the electric circuit by closing it. Such a device is described for instance in French patent No. 981,827. This type of device has certain drawbacks. One drawback is that, each time the rod changes its inclination upon a contact of short duration with the ground, for instance on a road which is in poor condition, it modifies the condition of the electric circuit and therefore sets off the alarm signal. Another drawback is that, in order to ensure the closing of the electric circuit under the effect of the inclination of the rod, it is necessary to provide relatively complicated mechanical means, which may get out of order.
Devices are also known in which the flexible rod is arranged at a slight distance from one sidewall of a tire and is deflected by the fact that when the inflation pressure of the tire decreases, the sidewalls of the tire curve more towards the outside. This deflection has the result that the sidewall of the tire which faces the flexible rod modifies the inclination of the rod, which then leaves its position of rest in which it keeps the electrical circuit closed. The circuit is then opened, which causes a signal to be generated to alert the driver.
A drawback of this known device is that it is difficult to use with radial tires since they have very flexible sidewalls which, even at normal pressure, deform a great deal near the area of contact with the ground, particularly when going around curves.